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Comment by wrs | Hacker News Reader

You still had to tell the compiler ahead of time about the property. The actual test is more like

    NSObject obj = [NSObject init];
    giveObjectAProperty(obj, @"progressView");
    obj.progressView = [NSNumber 42];

Writing giveObjectAProperty may not be "nearly impossible" but it sure isn't easy.

And of course the parent is talking about methods, not properties, but...same argument.

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Fair enough.

Here's how you'd make that class on the fly:

   Class mySubclass = objc_allocateClassPair([NSObject class], "MySubclass", 0);

Here's how you'd add a description method to a newly created class:

   static NSString *Description(id self, SEL _cmd){
        return [NSString stringWithFormat: @"<%@ %p: foo=%@>",[self class], self, [self foo]];
   }

    // add Description to mySubclass
    // grab NSObject's description signature so we can borrow it
    Method description = class_getInstanceMethod([NSObject class],
                                                 @selector(description));
    const char *types = method_getTypeEncoding(description);
    
    // now add
    class_addMethod(mySubclass, @selector(description), (IMP)Description, types);

And your iVars:

   class_addIvar(mySubclass, "foo", sizeof(id), rint(log2(sizeof(id))), @encode(id));

What jballanc is saying is that yes these things are possible in Objective-C, but the technique of doing these tricks in Ruby/Motion is an order of magnitude simpler and idiomatic:

    klass = Object.const_set "MySubclass", Class.new(Object)
    
    klass.send(:attr_accessor, :foo)
 
    klass.send(:define_method, "description") {
      "<#{self.class}: foo=#{self.foo}>"
    }

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